Known in the art is a single-crystal material based on yttrium-aluminium garnet (Y.sub.3 Al.sub.5 O.sub.12) containing chromium as a dyeing additive in an amount of 0.01 to 3% by weight. This material has a green colour and a red luminescence (cf. CSSR Pat. No. 152,908 Cl. B 01 J 17/02, published 08.12.71).
Also known is a single-crystalline Jewelry material based on yttrium-aluminium garnet, wherein as the dyeing additive titanium is used in an amount of from 1.10.sup.-4 to 1% by weight. This material has a clearly pronounced brown colour under daylight passing to a reddish colour at an artificial illumination (cf. USSR Inventor's Certificate No. 594628, Cl. B 01 J 17/00, Bulletin No. 17 published in 1978).
The use of these dyeing additives in the above-mentioned single-crystalline materials does not make it possible to obtain a red dyeing spectrum.